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Except that's not what the manufacturer states and they provide a very technical explanation (beyond my understanding) as to why this changes as current or VDD increases. I'm seeing the same characteristics in several different amplifiers using different transistors. It was so misleading in my early low power testing that I thought I would need to design the bias to increase with heat. What a disaster that was. The problem completely disappears once the bias is thermally tracking correctly too.

The downward drift in bias at very low drive is almost not measurable and is a tiny percentage of the total bias current that does not require any compensation. The drift upwards at normal power can reach self destructive levels if not tracked and regulated.

The upward drift in bias current is directly related to heat. I can control how high it rises by how long I hold the amp keyed with a full power carrier. When I remove the drive and maintain bias, I can clearly see that is has increased substantially and quickly drops back to normal as the junction cools. It was only when I made the mistake of holding it keyed too long and the DC bias drifted up very high, removal of drive could not break the thermal runaway cycle however, no RF at all could be seen on the scope because the additional current is all DC component.

<blockquote data-quote="Shockwave" data-source="post: 727052" data-attributes="member: 10882">Except that&#039;s not what the manufacturer states and they provide a very technical explanation (beyond my understanding) as to why this changes as current or VDD increases. I&#039;m seeing the same characteristics in several different amplifiers using different transistors. It was so misleading in my early low power testing that I thought I would need to design the bias to increase with heat. What a disaster that was. The problem completely disappears once the bias is thermally tracking correctly too.The downward drift in bias at very low drive is almost not measurable and is a tiny percentage of the total bias current that does not require any compensation. The drift upwards at normal power can reach self destructive levels if not tracked and regulated.The upward drift in bias current is directly related to heat. I can control how high it rises by how long I hold the amp keyed with a full power carrier. When I remove the drive and maintain bias, I can clearly see that is has increased substantially and quickly drops back to normal as the junction cools. It was only when I made the mistake of holding it keyed too long and the DC bias drifted up very high, removal of drive could not break the thermal runaway cycle however, no RF at all could be seen on the scope because the additional current is all DC component.</blockquote>

[QUOTE="Shockwave, post: 727052, member: 10882"] Except that's not what the manufacturer states and they provide a very technical explanation (beyond my understanding) as to why this changes as current or VDD increases. I'm seeing the same characteristics in several different amplifiers using different transistors. It was so misleading in my early low power testing that I thought I would need to design the bias to increase with heat. What a disaster that was. The problem completely disappears once the bias is thermally tracking correctly too. The downward drift in bias at very low drive is almost not measurable and is a tiny percentage of the total bias current that does not require any compensation. The drift upwards at normal power can reach self destructive levels if not tracked and regulated. The upward drift in bias current is directly related to heat. I can control how high it rises by how long I hold the amp keyed with a full power carrier. When I remove the drive and maintain bias, I can clearly see that is has increased substantially and quickly drops back to normal as the junction cools. It was only when I made the mistake of holding it keyed too long and the DC bias drifted up very high, removal of drive could not break the thermal runaway cycle however, no RF at all could be seen on the scope because the additional current is all DC component. [/QUOTE]

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